The E2Fs are a family of transcription factors that interact with the protein product of the tumor suppressor gene Rb and thereby play a critical role as regulators of cell proliferation and differentiation. To study the function of two members of the E2F family, E2F-1 and E2F-2, we have begun to generate cells and mice that lack these genes using the technique of targeted disruption by homologous recombination. To achieve these overall goals we propose the following specific aims: (1) To generate cells that lack functional E2F-1 or E2F-2 by gene disruption in mouse embryonic stem (ES) cells. In preliminary experiments this has already been achieved for E2F-2, and the experiments are underway for E2F- 1; (2) To examine the growth and differentiation of ES cell lines that contain heterozygous or homozygous disruptions of the E2F-1 or E2F-2 genes, and compare the properties of these cells with wild-type ES cells; (3) To produce transgenic mice that are heterozygous or homozygous for the disrupted E2F-1 or E2F-2 genes by blastocyst injection of ES cells in which these genes have been disrupted; (4) To examine the phenotype of mice with disrupted E2F-1 or E2F-2 genes with respect to their embryonic development and adult phenotype with particular emphasis on the development and function of blood cell lineages. Taken together, the results of these experiments have the potential to give important new information regarding the specific functions of E2F-l and E2F-2 during cell proliferation, differentiation, and oncogenesis.